UIImage *bgImage = [UIImage imageNamed:@"bg"];
    
    //上下文 : 基于位图(bitmap),所有的东西需要绘制到一张新的图片上去
    
    //1.创建一个基于位图的上下文(开启一个基于位图的上下文)
    //size : 新图片的此处
    //opaque : 不透明度
    //创建一张新的bitmap,也就是新的UIImage对象
    UIGraphicsBeginImageContextWithOptions(bgImage.size, NO, 0.0);
    
    //2.画背景
    [bgImage drawInRect:CGRectMake(0, 0, bgImage.size.width, bgImage.size.height)];
    
    //3.画右下角水印
    UIImage *watermarkImage = [UIImage imageNamed:@"watermark"];
    CGFloat scale = 0.2;
    CGFloat margin = 5;
    CGFloat watermarkW = watermarkImage.size.width * scale;
    CGFloat watermarkH = watermarkImage.size.height * scale;
    CGFloat watermarkX = bgImage.size.width - watermarkW - margin;
    CGFloat watermarkY = bgImage.size.height - watermarkH - margin;
    
    [watermarkImage drawInRect:CGRectMake(watermarkX, watermarkY, watermarkW, watermarkH)];
    NSLog(@"%g",watermarkY);
    
    //4.从上下文中取得制作完毕的UIImange对象
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    
    //5.结束上下文
    UIGraphicsEndImageContext();
    
    //6.将image对象压缩为PNG格式的二进制对象.
    NSData *data = UIImagePNGRepresentation(newImage);
    
    //7写入文件
    NSString *path = [[NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingString:@"new.png"];
    [data writeToFile:path atomically:YES];
    NSLog(@"%@",path);

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